Presidential pardons normally come up at the end of a presidency when a president is on the way out and less worried about controversy. But President Trump began using his pardon power the very first summer he was in office and has hinted at using it in a variety of sensitive cases now pending. This is the sort of historical issue we like to toss to Ron Elving, NPR's senior editor and correspondent on the Washington Desk, the man we call Professor Ron.

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RON ELVING, BYLINE: The most famous presidential pardon was granted by one president to another. It was 1974, and Richard Nixon had just become the only president ever to resign.

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RICHARD NIXON: To leave office before my term is completed is abhorrent to every instinct in my body.

ELVING: Submitting to the reality of pending impeachment.

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NIXON: I shall resign the presidency, effective at noon tomorrow. Vice President Ford will be sworn in as president at that hour in this office.

ELVING: Exactly one month later...

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GERALD FORD: I, Gerald R. Ford...

ELVING: ...The nation would hear this stunning announcement on a Sunday morning.

ELVING: Nixon had been named a co-conspirator in the Watergate political spying scandal. But with a pardon, he could never be charged or held to account for obstruction of justice or any other crimes. That decision hung over Ford's presidency and probably cost him the next election. The basic pardon power is in the Constitution and goes back to the Founding Fathers, who saw it as useful - redeeming the wrongly convicted and for political gamesmanship, much as the pardon power had been for kings in England and elsewhere.

George Washington, on his final day in office, pardoned two condemned men who had participated in the Whiskey Rebellion, exercising what he called his sacred duty...

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UNIDENTIFIED PERSON #1: ...Sacred duty to mingle in the operations of government every degree of moderation and tenderness which the national justice, dignity and safety may permit.

ELVING: President Andrew Johnson issued a pardon of Civil War rebel soldiers and leaders, including Confederate President Jefferson Davis, on Christmas Day, 1868. In the 20th century, pardons were no less entwined with politics.

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UNIDENTIFIED PERSON #2: We got the first details today on what Jimmy Carter's plans are for pardoning Vietnam draft resisters and evaders.

ELVING: President Carter, on his first full day in office in 1977, issued a blanket pardon to all Vietnam War resisters, a promise he had made during his campaign. President George H.W. Bush...

ELVING: ...Some of the former officials who had been involved in the Iran-Contra affair more than five years earlier under previous president Ronald Reagan. And President Clinton pardoned a financier named Marc Rich, who was a fugitive from tax charges.

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UNIDENTIFIED PERSON #4: Rich had been living in Switzerland. He fled there in 1983 to avoid charges of breaking U.S. sanctions by selling Iranian oil to, among others, the apartheid regime in South Africa.

ELVING: Rich had contributed to the Clinton Presidential Library, which inflamed both Democrats and Republicans. But Clinton did not issue his first pardon until he had been in office nearly two years. President George W. Bush waited longer than that and President Obama still longer yet. President Trump, in his first 17 months in office, has already issued two pardons.

UNIDENTIFIED PERSON #6: We're just getting this statement in from the White House. The president has pardoned Scooter Libby, the former chief of staff to then-Vice President Dick Cheney. A lengthy statement...

ELVING: Legal scholars agree on at least a few limits to the pardon power. One is that the president cannot pardon people for crimes not yet committed. Another is that a president cannot pardon himself because he cannot act as judge on his own case. But on Twitter, Trump has said that, quote, "all agree the president has complete power to pardon," unquote. It's unclear what he means by complete. But when asked about using that pardon for others, such as his former campaign chairman Paul Manafort who's been indicted by special counsel Robert Mueller, the president is noncommittal.

ELVING: Each new president has used his pardon powers in his own way. But pardoning former associates who might otherwise flip or testify against the president would be breaking new ground. Ron Elving, NPR News, Washington.

(SOUNDBITE OF JOHN ADAMS' "THE CHAIRMAN DANCES") Transcript provided by NPR, Copyright NPR.